Air compressors are used for a variety of applications which include, but are not limited to, air brake systems for over-the-road commercial trucks. Engineers are constantly seeking ways to improve air compressors in an effort to increase their useful life and durability, thereby reducing maintenance costs and system downtime.
One problem associated with conventional air compressor designs occurs when the compressor is run constantly causing the temperature of the air compressor exhaust gas to rise to levels that may be damaging to air compressor operation. To overcome this problem, some air compressors are run in loading and unloading cycles so that the air compressor does not overheat. Normally, an air compressor operates in a loading state where air is pumping into an adjacent wet tank. Upon certain predetermined conditions, the compressor unloads causing air to be pumped into the atmosphere.
An example of a conventional air compressor which operates in a manner similar to that described above is disclosed in U.S. Pat. No. 1,068,432 to Hill which teaches an air compressor control that utilizes a thermally actuated means to control the unloading of the air compressor to eliminate danger of an accident and place the compressor in a safe condition to be shut down. In order to accomplish this objective, Hill proposes that when the compressor becomes superheated, a valve is opened so that air within the compressor is exhausted directly from the cylinder to the atmosphere. This instantly relieves the pressure in the cylinder and stops the accretion of heat. Although the air compressor system of Hill discloses a method of protecting the air compressor from damage, the system of Hill merely unloads the compressor if a certain predetermined temperature is reached and is not dependent on wet tank pressure thresholds to yield efficient compressor loading and unloading operation.
U.S. Pat. No. 2,052,168 to Crittenden discloses a compressor protection device that is responsive to temperature conditions of the fluid compressed by a multicylinder compressor and includes a controller for unloading the compressor based on compressor operating conditions. When the pressure of the fluid within a reservoir adjacent the compressor reaches a predetermined value or upon the occurrence of other conditions to which the unloading means is responsive, the compressor is unloaded. The compressor remains unloaded until the unloading device of the compressor operates to effect loading. If the fluid compressed in a cylinder exceeds a predetermined value, then the cylinder is unloaded. The device of Crittenden provides for compressor safety and protection and may be effective in certain environments, however, the device does not appear to effectively control air compressor operation based on both sensed air compressor exhaust gas temperatures and an optimum wet tank pressure range to ensure efficient air compressor operation. In essence, the Crittenden device only relies on a limited number of conditions, such as one temperature limit or one pressure limit, to determined whether the compressor should be loaded or unloaded.
Another system disclosed in U.S. Pat. No. 3,961,862 to Edstrom et al. appears to provide a more efficient means of operation than the two systems discussed above. Edstrom et al. provides a compressor control system for unloading gas compressors in a way that reduces back pressure or working pressure in the discharge port and the compressor working chambers. The compressor of Edstrom et al. is arranged in a circuit with a closable chamber disposed upstream of the compressor gas inlet. A control circuit is operable to sense compressor discharge pressure and at a predetermined pressure condition, sequentially operate multiple valves to shut off compressor inlet flow, thereby evacuating the chamber and placing the compressor discharge port in communication with the chamber. In this mode, the compressor runs unloaded or at idle at a greatly reduced inlet and discharge pressure. By sensing the chamber vacuum or by the use of time delay devices, the unloading of the compressor may be controlled. Although the Edstrom et al. system is an improvement over the Hill and Crittenden devices with respect to the number of conditions used to determine whether a compressor should be loaded or unloaded, the system is rather complex in nature and thus, would appear to be very costly to manufacture and maintain.
In view of the references discussed above, there is clearly a need for a compressor control system that is inexpensive to manufacture and maintain, while providing effective and efficient compressor operation to improve the durability of the compressor and its useful life.